Some automatic transmissions include a low/reverse braking clutch (i.e., a friction brake) that is applied in both a reverse speed ratio and a first forward speed ratio in order to provide reaction torque in both forward and reverse directions. Alternatively, a braking clutch is sometimes used alongside a one-way clutch, with the one-way clutch preventing rotation of a transmission component, such as a carrier member, in either the first or reverse speed ratio, and the braking clutch applied to prevent rotation in the other of the first and the reverse speed ratio. This type of one-way clutch is not selectable, in that it always automatically brakes one direction of rotation of the transmission component and freewheels, i.e., permits the component to rotate, in the opposing direction.
Automatic, multi-speed transmissions that incorporate planetary gear sets typically include multiple friction-type clutches in order to manipulate the transmission to provide many forward speed ratios. Many such components are necessary to provide six, seven, or eight forward speed ratios. The planetary gear sets and clutches become increasingly difficult to package as they increase in number, and also add to the overall weight of the transmission.
As is commonly known to those familiar with the art, disengaged multi-plate friction clutches, depending on various conditions, produce drag when there is relative motion between the sets of clutch plates. One characteristic of a disengaged multi-plate clutch is that, as the relative speed between the sets of clutch plates increases, typically, so does the drag or spin loss. This spin loss contributes to decreased fuel economy.